The present invention relates to a system which permits high speed, high resolution inspection of printed webs.
In many industries packaging materials are printed at extremely high rates of speed up to 1200 feet per minute. Some automatic method of detecting flaws in the printed web must be employed since human inspectors cannot discern errors, other than gross errors, at such rates of speed. This invention permits the detection of misprinted areas in a printed web at the rate of speed of printing presses used in the printing of packaging for consumer products, for example cigarette packs.
It is known that flaws in webs of printed fabric can be detected by passing light through the fabric, sensing the light electrically, converting the amount of light sensed into a proportional voltage signal and then comparing the input to the standard signal for flawless fabric, as disclosed in U.S. Pat. No. 4,952,062. However, these types of systems do not operate at the high rates of speed needed in the packaging industries. Also, these systems do not provide a means for the detailed analysis of printed patterns on more opaque heavy coated paper stock web.
Inspection systems that convert the printed matter to video images, and then digitally process the video images, are also known. However, the difficulty of detecting flaws in images at such a high rate of speed is the amount of data generated for an entire image. The use of algorithms which detect flaws by comparing an entire image to a predetermined pattern requires too much processing power to be cost effective in a real-time operation.
A printed web which is 102.4 millimeters (or approximately 4 inches) wide would require 1 kilobyte (1024 bytes) of data per video scan line in order to detect flaws of 0.1 millimeter. Each 0.1 millimeter square could be considered one pixel. Each pixel requires 8 bits of binary data (1 byte) to represent the 256-level gray scale value assigned to describe the reflectivity of each pixel. At 500 feet per minute, 25,400 lines at a resolution of 0.1 millimeter pass every second. Therefore, the total data throughput at 500 feet per minute is approximately 25,400 kilobyts per second (24.8 megabytes per second). For complex pattern recognition algorithms, this rate of data throughput is too great to permit real-time recognition of flawed patterns with currently available cost effective technology.
Another difficulty with such web speeds is the inherent jitter of the web in both the longitudinal and transverse axes. Longitudinal jitter results from longitudinal movement of the web which is greater than the movement detected by the shaft encoder (i.e., a phase change between the web and the shaft encoder). This longitudinal "slipping" can occur due to wearing of the mechanical components of the web transport apparatus. Transverse jitter is the movement of the web perpendicular to the direction of movement of the web. Transverse jitter occurs due to the inherent difficulties in providing exact tolerances in a mechanical web printing apparatus. Such jitter causes the input image of the inspection system to be misaligned with respect to the target image resulting in incorrect flaw determinations.
In the consumer products packaging industries a flexible system is also required. Since label requirements are often dictated by marketing needs or federal and state administrative agencies, a system which is easily trained to detect flaws in different label configurations is necessary. In the cigarette industry the four Surgeon General's warning labels rotate on a quarterly basis. Reprogramming, or "training", of a web inspection system to accommodate changes in the printed matter must be reduced to a minimum to deal with such frequent changes.
It would be desirable to provide a web inspection system which is capable of high speed inspection of webs to provide for the real-time detection of flaws in printed webs produced at high speeds--e.g., up to about 1200 feet per minute.
It would also be desirable to provide a web inspection system capable of being trained quickly and easily on different image sets.
Furthermore, it would be desirable to provide a web inspection system which is capable of minimizing the effect of misalignment of input images due to jitter in the longitudinal and transverse axes.
It would further be desirable to provide a web inspection system that accepts or rejects inspected webs based on statistical rules ascertained through prior inspections.
Finally, it would be desirable to provide a web inspection system which is an expandable modular design using cost-effective available hardware.